Re: VCE exceptions

["William J. Earl" <wje@fir.engr.sgi.com>]

ralf@uni-koblenz.de writes:
 > I think I know why we're catching VCE exceptions even though we try to
 > avoid them at any price - the reason spells ``empty_zero_page''.  This
 > page is filled with zeros and is being mapped to arbitrary addresses
 > at the same time.  Arbitrary addresses means also bits 14:12 of the
 > virtual address may be different, welcome VCED.  This also means that
 > at least sane code should never cause VCEI exceptions.  The text of
 > the panic message ``should not happend'' is therefore wrong as well ...
 > 
 > Whatever, the fact that the hardware causes VCE exceptions which don't
 > help us at all forces us to handle them somehow.  How handy, they'll
 > fit quite well in the revamped interface for board caches :-)
 > 
 > Another way to finally eleminate the virtual coherency problem from
 > KSEG0's landscape would be to actually use 8 pages as an array of
 > empty_zero_pages[], so we would be able to map one wherever we want
 > such that we never run into virtual coherency trouble.

      For an always-zero page, this is the best solution.  At a small
cost in memory, you get far less overhead.